This invention relates to improvements on a color television cathode ray tube and more particularly to improvements on phosphors applied to the surface of the glass face plate of the cathode ray tube.
Generally, a color television cathode ray tube comprises a glass face plate whose surface is coated with a phosphor screen or film formed of dotted or striped red-, green- and blue-phosphors. Various colors are reproduced according to the different degrees to which electron beams emitted from three electron guns through a shadow mask excite the respective phosphors. To date, the phosphor screen has been formed of a red-emitting phosphor of europium-activated yttrium oxide (Y.sub.2 O.sub.3 :Eu) or europium-activated yttrium oxy-sulfide (Y.sub.2 O.sub.2 S:Eu), a green-emitting phosphor of copper-activated zinc cadmium sulfide (ZnCdS:Cu), and a blue-emitting phosphor of silver-activated zinc sulfide (ZnS:Ag).
Since, however, the cadmium contained in the green-emitting phosphor is an element extremely toxic to the human body and gives rise to environmental pollution, a cadmium-free copper-activated zinc sulfide has recently come into practical use. FIG. 1 presenting the CIE chromaticities of various color-producing systems proves that this cadmium-free green-emitting phosphor displays, as indicated by a dot G.sub.2, a green color less blended with a reddish hue than the prior art green-emitting phosphor of copper-activated zinc cadmium sulfide represented by a dot G.sub.1. For reproduction of a white color in a region having a relatively low color temperature, therefore, there has been raised the problem of unavoidably introducing an excess amount of current through a red-emitting electron gun. With IR, IG and IB taken to denote the different amounts of current flowing through the cathodes of the respective electron guns to excite the red-, green- and blue-emitting phosphors for reproduction of a white color of, for example, 6500.degree. K.+7 MPCD, then the combinations of the phosphors and the ratios of the amount of current running through the cathodes of the electron guns have relationship set forth in Table 1 below. It is seen from Table 1 that where a copper-activated cadmium-containing green-emitting phosphor (ZnCdS:Cu) is used, the ratio of IR/IB is 1.66 as against an exceedingly high IR/IB ratio of 2.40 where a cadmium-free green-emitting phosphor is applied, obviously causing a red color-exciting electron gun to have an unduly high load.
______________________________________ Relative brightness Blue- Green- Red- of a emitting emitting emitting white phosphor phosphor phosphor IR/IB IR/IG color ______________________________________ ZnS:Ag ZnCdS:Cu Y.sub.2 O.sub.2 S:Eu 1.66 1.02 100 ZnS:Ag AnS:Cu Y.sub.2 O.sub.2 S:Eu 2.40 1.26 92 ______________________________________
In case of a cathode ray tube embodying this invention
______________________________________ Zns:Ag ZnS:Au + ZnS:Cu Y.sub.2 O.sub.2 S:Eu 1.72 1.01 99 ______________________________________
Table 1 above shows that where a cadmium-free copper-activated green-emitting phosphor (ZnS:Cu) is applied, red color-exciting electron beam is focused less satisfactorily, and also less acceptable in consideration of the life of an electron gun. For comparison Table 1 also presents data on a color television cathode ray tube embodying this invention.
The object of this invention is to provide a color television cathode ray tube which comprises a cadmium-free green-emitting phosphor and can decrease the ratio of amounts of cathode currents of red and blue electron guns, IR/IB.